1. Field of the Invention
The present invention relates to a printer having a head, which provides inks of different densities with respect to at least one hue and may create at least two different types of dots having different ink quantities with respect to each of the different density inks, and enabling a multi-tone image to be printed with inks spouted from the head. The present invention also pertains to a method of printing as well as a recording medium for implementing the method.
2. Description of the Related Art
Color printers that spout multiple colors of inks from a head are widely used as an output device of a computer, which prints images processed by the computer in a multi-color, multi-tone manner. In order to further improve the printing quality in an area of low image density, that is, a highlighted area, a printer and a printing method using higher density and lower density inks have been proposed. This technique provides both a higher density ink and a lower density ink for an identical hue and controls the spout of these inks, so as to attain printing of the excellent tone expression.
Another proposed printer for multi-tone expression creates two different types of dots having different ink densities and ink quantities and thereby changes the density per unit area in multiple stages (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 59-201864). In this printer, one pixel consists of four dots. The technique applied to this printer changes the frequency of appearance of the higher-density dots and the lower-density dots in the pixels and thereby enables an image to be printed with multiple densities.
In the printer that spouts ink and creates dots, the quantity of ink per unit area, that is, the ink duty, should be controlled not to exceed a preset level, which depends upon the type of printing paper. The spout of ink exceeding this preset level causes the printing paper to be readily broken and makes ink stains on the printing paper, thereby lowering the picture quality of resulting printed images.
In the prior art printer that enables creation of two different types of dots having different ink densities and ink quantities, there is no specific consideration for the ink duty. The prior art printer simply creates these two different types of dots according to the input tones of the respective pixels based on a preset pattern, and does not take into account the restriction of ink duty nor attain the well-balanced regulation of the quantities of inks having different densities to create the different types of dots.
The object of the present invention is thus to provide a technique applicable to a printer, which enables at least two different types of dots having different ink densities and ink quantities to be utilized effectively when there is a requirement in regulation of the ink quantities for dot creation, for example, when there is a restriction of ink duty.
At least part of the above and the other related objects is attained by a printer that creates a plurality of dots and thereby prints an image on a printing medium. The printer includes: an input unit that inputs image data for each of pixels included in an original image; a head that provides a plurality of inks, which include at least two inks of different densities with respect to at least one hue, and enables creation of at least two different types of dots having different quantities of ink for each of the plurality of inks; an expected ink quantity setting unit that sets an expected quantity of ink, which is to be spouted for creation of a dot, in each pixel based on the image data, with respect to each of the plurality of inks; a multi-valuing unit that sets an on-off state of each of the at least two different types of dots in each pixel, based on the expected quantity of ink, with respect to each of the plurality of inks; and a dot creation unit that creates dots set in the on state.
In the printer of the present invention, the technique sets the expected quantity of ink used for creation of a dot in each pixel, with respect to each of a plurality of inks, which include at least two inks having different densities provided for at least one hue, and determines the type of the dot to be created in each pixel. This arrangement enables the quantities of ink used for creation of dots to be regulated appropriately.
The expected quantity of ink here represents an ideal quantity of ink to be spouted in each pixel for expression of the tone value included in the input image data. In the printer of the present invention, the quantities of ink used for creation of dots are practically restricted to several values. The expected quantities of ink are, however, not restricted to these several values but may take any continuous values in a specific range. Although the expected value is a term generally used in the field of probability, the term xe2x80x98expected value or quantityxe2x80x99 in this specification means a value or quantity of ink expected to spout in each pixel.
The expected quantity of ink may not coincide with a quantity of ink actually required for expressing the tone value of the image data but may be determined by taking into account the characteristics of the nozzles, from which ink is spouted, and other related factors. The expected quantity of ink may be greater than or less than the quantity of ink actually required for expressing the tone value of the image data. By way of example, the expected quantity of ink may be set greater for some tone values and set smaller for other tone values.
In accordance with one preferable application of the printer, the image data includes tone values with respect to at least one color component in a color space. The expected ink quantity setting unit includes: an expected values storage unit that stores expected quantities of inks corresponding to combinations of tone values regarding a plurality of color components in the color space in the form of a table; and a unit that refers to the table, which is stored in the expected values storage unit, and reads an expected quantity of ink in each pixel corresponding to the input image data.
The expected quantities of inks to be spouted in the respective pixels are determined according to the combinations of the tone values regarding the plurality of color components in the color space and stored in the form of a table. The arrangement discussed above enables the expected quantity of ink to be set in each pixel, based on this table.
In the printer of the above configuration, it is preferable that the table stored in the expected values storage unit specifies expected quantities of each color ink, which are determined by taking into account expected quantities of the other color inks irrespective of the hue, corresponding to the combinations of the tone values regarding the plurality of color components in the color space.
In the table stored in the expected values storage unit, the expected quantities of each color ink are determined by taking into account the expected quantities of the other color inks. This arrangement ensures regulation of the total quantity of ink and improves the picture quality of a resulting image. There is typically an upper limit in quantity of ink to be spouted per unit area. For example, it is assumed that the printer records an area of a specific density with two inks of different densities, that is, the light ink and the deep ink, for cyan. When the expected quantities of inks are small for the other hues, there is a sufficient margin to the upper limit. In this case, the expected values are set to use a large quantity of the light ink, which makes cyan dots relatively inconspicuous. When the expected quantities of inks are large for the other hues, on the other hand, there is an insufficient margin to the upper limit. In this case, the expected values are set to decrease the quantity of the light cyan ink and increase the quantity of the deep cyan ink.
A variety of methods may be applied to the multi-valuing unit in the printer of the present invention. Some examples of the available method include a dither method and an error diffusion method.
A single multi-valuing unit may be used for all the inks or alternatively separate multi-valuing units may be provided for the respective inks.
When the number of the different types of dots created by the printer is limited, the expected quantity of ink may not coincide with the quantity of ink actually used for creation of a dot. There is accordingly an error in ink quantity in each pixel. The printer of the above arrangement carries out the multi-valuing process to reduce such errors in the image as a whole, although there are errors occurring in the respective pixels. The dither method enables the high-speed multi-valuing process, whereas the error diffusion method appropriately reduces the errors to attain the favorable picture quality of resulting images.
In accordance with another preferable application of the printer, the multi-valuing unit includes: an expected value-by type setting unit that sets an expected value by type, that is, an expected value of ink quantity by each of the at least two different types of dots, which may be created by the head, based on the expected quantity of ink set by the expected ink quantity setting unit; and a unit that determines creation or non-creation of a dot in each pixel with respect to each of the at least two different types of dots, based on the expected value by type.
The printer of this arrangement sets the expected value of ink quantity by each type of the dot and regulates the occurrence of each type of the dot, in order to eliminate the errors in the image as a whole. This arrangement causes the dots of each type to be dispersed in a resulting image in a substantially homogeneous manner. The dots having the greater ink quantity are generally more conspicuous. A local polarization of such dots significantly deteriorates the picture quality of the resulting image. The arrangement causes even the dots having the greater ink quantity to be dispersed in a substantially homogeneous manner, thereby attaining the good picture quality of the resulting image where the dots are not inconspicuous.
In the printer of the present invention, it is preferable that dots created for an identical hue by the head include at least two different types of dots having a substantially identical mean density per unit area at one recording density.
When there are at least two different types of dots having a substantially identical mean density per unit area, there is a freedom in selection between these dots at a specific recording density that can record an image with these dots. This arrangement accordingly enables these dots to be created at an appropriate rate, in order to improve the picture quality and satisfy a variety of other requirements, while regulating the quantities of inks. The relationship between the mean density per unit area and the recording density of dots generally depends upon the type of the dot. In order to attain the effects discussed above, there should be at least one recording density, at which the at least two different types of dots have a substantially identical mean density.
In the printer of the present invention, it is also preferable that the expected ink quantity setting unit sets the expected quantity of ink in a specific range that does not cause a sum of ink quantities spouted for creation of dots per unit area irrespective of the hue to exceed a preset level, which depends upon the printing medium.
The printer of this arrangement regulates the sum of the ink quantities per unit area not to exceed a preset level, which is determined according to the type of the printing medium. The printing medium generally has an upper limit in absorbable quantity of ink. The spout of ink exceeding this upper limit causes the printing medium to be readily broken and makes ink stains on the printing medium, thereby significantly deteriorating the picture quality. The printer of the above arrangement regulates the sum of the ink quantities not to exceed the upper limit and thereby avoids such problems that lead to the deteriorating picture quality.
The preset level, which depends upon the type of the printing medium, is a quantity of ink set by taking into account a variety of factors of the printing medium, for example, the stain characteristics of the printing medium, the time required for drying the ink, the picture quality of the printed image, and the deformation of the printing medium due to absorption of ink. The preset level is not unequivocally determined by the printing medium, but also depends upon the printing speed and the color of the ink used for printing. The expected quantity of ink determined by considering such an upper limit should be different from an ideal quantity of ink set to attain the favorable picture quality when there is no restriction in ink quantity. One possible application of setting the expected quantity of ink by considering the upper limit changes the expected quantity of ink corresponding to an identical tone value between the normal printing paper and the special printing paper, which is manufactured for the purpose of the high-quality printing and absorbs a less amount of ink.
The preset level is the sum of ink quantities per unit area and does not prohibit the ink quantity from locally exceeding the preset level. For example, when the ink quantity exceeds the preset level with respect to a specific pixel, the requirement is to decrease the quantity of ink spouted in the vicinity of the specific pixel and cause the sum of the ink quantity not to exceed the preset level in the unit area as a whole.
In the printer of the present invention, the head used to create dots with inks of different densities or create dots having different ink quantities may have a mechanism for spouting an ink particle with a pressure applied to ink in an ink conduit by application of a voltage to a piezoelectric element disposed in the ink conduit. The head may alternatively have a mechanism for spouting an ink particle with a pressure applied to ink in an ink conduit by bubbles produced by supply of electricity to a heating element disposed in the ink conduit. These mechanisms facilitate the formation of finer ink particles and the appropriate regulation of the ink quantities. These mechanisms also allow a large number of nozzles to be formed on the head. In the latter case, the large number of nozzles for spouting ink particles are arranged in the feeding direction of the printing paper for the respective inks of different colors and different densities. The use of the large number of nozzles improves the printing speed.
A possible application of the present invention other than the printer is a method of printing. Another possible application of the present invention is a recording medium, in which a program used for driving the printer having any one of the arrangements discussed above or data used in the program are recorded.
Typical examples of the recording media include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with bar codes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. Still another application of the invention is a program supply apparatus that supplies the program to the computer via a communications path. In this case, the program for driving the printer may be supplied collectively as a whole or in the unit of each module.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment with the accompanying drawings.